/Engine/Plugins/Developer/Concert/ConcertMain/Source/Concert/Public/IConcertSession.h
Register a custom event handler for this session
[FDelegateHandle](API\Runtime\Core\Delegates\FDelegateHandle) InternalRegisterCustomEventHandler ( const [FName](API\Runtime\Core\UObject\FName) & EventMessageType, const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [IConcertSessionCustomEventHandler](API\Plugins\Concert\IConcertSessionCustomEventHandle-) > & Handler )